Numerical control positioning servosystem



Dec. l1, 1962 J. J. JAEGER ET Al.

NUMERICAL CONTROL PosIToNING sERvosYsTEM Filed Aprile, 1960 5Sheets-Sheet l ATTORNEYS Dec. 11, 1962 J. J. JAEGER ET AL 3,068,386

NUMERICAL CONTROL PosIToNING sERvosYsTEM Filed April 8, 1960 5Sheets-Sheet 2 (TRUE NULL) F l G. 2 36 (74 (T4 f A I 465// '\46b JACOBJ. JAEGER EDWARD E. KIRKHAM BYEDWARD F. NOWAK mm f/0%@ I ATTORNEYS Dec.11, 1962 J. J. JAEGER I-:TAL

NUMERICAL CONTROL POSITIONING sERvosYsTEM 5 Sheets-Sheet 5 Filed April8, 1960 INVENTOR` JACOB J. JAEGER E. KIRKHAM EDWARD BY EDWARD F. NowAKagamm 54M@ ATTORNEYS Dec. 11, 1962 J. J. JAEGER ET AL 3,058,386

NUMERcAL CONTROL POSITIONING sERvOsYsTEM Filed April 8, 1960 T 5Sheets-Sheet 4 TRUE NULL FALSE NULL FIRST OFFSET FALSE NULL SECONDOFFSET FALSE NULL SECOND OFFSET TRUE NULL FIRST OFFSET TRUE NULL TRUENULL FIGS JACOB J. JAEGER EDWARD E. K|RKHAM BY EDWARD F. NDWAK ATTORNEYSDec; l1, 1962 J. J. JAEGER ET AL NUMERICAL CONTROL POSITIONINGSERVOSYSTEM Filed April s, 1960 FIG? 5 Sheets-Sheet 5 JACOB J. JAEGEREDWARD E. KIRKHAM BY EDWARD F. NOWAK ATTORNEYS 3,0d8l6 NUMERCAL CONTRGLPOSTiNNG SERVSYSTEM Jacob J. Jaeger, North Canton, Edward E. Kirirham,Manchester, and Edward F, lslowalr, Hartford, Conn., assignors to Prattl' Whitney Conni-rarity, line., West Hartford, Conn.

Filed Apr. 8, 196%, Ser. No. 25,395

This invention relates to a digitally controlled positioning system formachine tools and the like. More particularly, it relates to an improvedposition sensing unit adapted to provide accurate information relatingto the position of a Worktable, carriage, or other movable element of amachine, as Well as an output signal useful in controlling the movementot the element into a preselected position. The sensing unitincorporates a magnetic head in a novel bridge circuit capable ofemitting null balance signals ,which control the inal approach speed otthe moving element and stop it at its desired position. Through the .useof unique logic circuitry, we have also eliminated errors resulting fromfalse bridge null indications.

Positioning systems of the general type with which our invention isconcerned make use of control information recorded in digital form onpunched tape, magnetic tape or other ,suitable media. An entire sequenceof machine operations may be recorded in this manner 'for automaticcontrol of the machine. As an example of the manner in which the variousmachine movements are controlled, one may consider the traverse of aWorktable on a jig borer. The workpiece to be bored is clamped to thetable and the table is moved so that holes may be bored at Variousspecified locations on the workpiece, as dictated by the recordeddigital information. The Worktable is mounted on a slide guided fortraverse on a movable carriage. The carriage is similarly mounted formovement in a direction perpendicular to the traverse of the Worktable,and thus by coordinating movement of the carriage and worktable, anypoint of the workpiece may be brought into position for boring.

In Aorder to control the location ci the Worktable on the carriage, aposition sensing system is provided to determine the position of thetable with respect to a reference point on the carriage. This positionis compared with the desired position obtained from the recordinfrmedium, a table drive unit is energized to move the table in the properdirection to make the actual position coincide with the desiredposition.

in one positioning system, a coarse control is used to bringtheWorktable Within a prescribed distance from its iinal position, and atine control then taires over to late movement over the final incrementof travel reguthe tine control always operates over a short distance, itmay be made to perform with a high degree oi accuracy; and the coarsecontrol is then relieved of rigorous accuracy requirements which areditlicult to obtain over a long range ot travel.

A typical coarse control uses a conver to convert the digital positioninstructions to an analog signal in the form of a voltage. The voltageis compared with a voltage from a position transducer reflecting theactual position of the turntable. riChe difference between these voltageis an error sional indicative of the distance between the actual anddesired positions, and the table is driven in the proper direction toreduce the error signal to zero.

The line control described herein uses a magnetic sensing head to sensethe position of the Worlrtable during its ilnal increment of travel. Ameasuring bar aligned with the direction of table traverse is mounted onthe worktable, The bar is provided with a series of accurately rn"-chined lands and indentations periodically spaced aicng its tati gdPatented Dec. lll, 1962 fi f' sie length. r.the sensing head, which ismounted on the carringe, has a pair of inductors whose iiux paths extendinto the measuring bar on the table. When one of the lands on themeasuring bar is positioned an equal distance from each of theinductors, their iuductances are equal, and a bridge circuitincorporating the inductors registers a null, herein termed a true null,to indicate this condition. The coarse positioning system moves thetable to bring a given land Within the magnetic field of the sensinghead, and the output of the sensing head is then used to control thefinal increment of table travel to bring it to rest in a true nullposition.

Economical use of machine tools dictates high speed for the variousoperations including traverse of the Worktablcs, carria ges, etc.However, ii the high speed is continued into the iinal increment oftravel, during which an accurate sensing unit measures the position ofthe moving element, inertia will cause the element to coast through thedesired nal position after the motive power has been cut oi'i.rherefore, it is desirable to derive a signal at a predetermined pointshortly prior to arrival at the linal position and use this signal toreduce the drive speed to a low level. Then when the position sensingunit indicates the close proximity of the moving element 'to its finalposition by means of a bridge null, as `described above, or othersuitable means, the driving power may be cut oit and the element willcoast approximately to the center of the dead band in which the errorsignal indicating departure from the exact null is insuilicient toreposition the element.

One method of obtaining the speed reduction signal is to use a sensinghead of the above type and connect additional impedance elements intothe bridge circuit incorporating the sensing head inductors. Theseelements change the balance condition of the bridge so that it balancesat a position reached by the moving element prior to the nal true nullposition. The null obtained at this prior position may be termed anoiset true null. When the offset true null position is reached, the nullsignal from the bridge circuit is used to disconnect the additionalimpedance elements from the bridge circuit as Well as reduce the speedof the moving element. Thus, the bridge once again becomes unbalanced toindicate positional error until the true null position is reached.

in order to obtain a complete null, i.e., substantial bridge balance, atthe oiiset truc null position, both thc magnitudes and phase angles ofthe various impedances in the bridge must be properly matched when theadditional elements use for the oifset null are connected into thecircuit. Prior to our invention this required the use of a substantialnumber of additional elements requiring careful adjustment. Furthermore,it is sometimes desirable to vary the position of the oiiset null over asubstantial range, and this has been di'thcult to accomplish with priorcircuits.

The use of magnetic sensing head of the above type has been accompaniedby complications resulting from bridge nulls which occur at measuringbar locations other than the respective true null and oiiset true nullpositions. More speciiically, a null, termed a false null, may occurwhen a groove, rather than a land, is between the inductors of thesensing head. In other Words, the false nulls occur iidway betweenadjacent true null positions of the measuring bar, and when the bridgecircuit connected to the sensing head is in its normal condition, thereWill be two nulls for each full unit of movement determined by thedistance between the centers of adjacent measuring bar lands.4 When theadditional circuit elements are connected to the bridge to place it inan offset condition, it wilt, of course, .register neither the true norfalse hulls. Rather, it will balance at both the oiset null position andan offset false null position reached shortly after the meassensingunit. It requires that only one additional element can be connected tothe bridge circuit to place the it in an offset condition. Theconnection is a parallel connection and is therefore easily made.Furthermore, the circuit can be readily adjusted to facilitate changesin the oiset null positions.

Turning now to FIGURE l, a worlttabie generally indicated at is suitablymounted for traverse on a carriage generally indicated at i2. Thevvorktabie and carriage may illustratively be used to position aworkpiece to be bored vbyva jig borer. The workta'ole it? is providedwith a slide schematically indicated at ifi, suitablyintertlttingwith'the top of the carriage i2. Traversing movement isimparted to the table lo by a table drive unit i6 connected to the tableby a shaft i8. The carriage i2 is conventionally provided with a slide(not shown) moua1 for movement in a direction perpendicular to the dtion of movement of the table liti. Thus, a workpiece clamped to thetable lo may be moved to any desired 4location by a suitable combinationof movements of the table 11i) (right and left FGY RE l) and me lage l2(toward and away from the reader). Co iol of traverse .of the table lilis accomplished lby a posi oning system described in detail below. Itwili be understood that control of lthe carriage i2 may be accomplishedin a similar manner.

Illustratively, the instructions for operation of the `rua-chime may berecorded on tape in which punci ed `holes are used to register therecorded information in a suitable binary code. Position instructionsare taken from the tape by 4a tape reader' 2@ whose output is connectedto a master control unit 22. An output of the control unit 22 isconnected to a digital-to-analog converter whose output provides oneinput of a summing unit The other input of the summing unit 24 is from aposition transducer 25 which provides a voltage indicative of the actualposition of the .table it?. The transducer 25 may take the form of ahelical potentiometer mechanically coupled to a rotating member in thedrive unit i6.

The error signal appearing at the output ofthe su- ,ming unit 2d is fedto an input 25a of a table control unit 26. The control unit 26 controlsmotors in the drive unit i6 as `well as gear arrangements used to couplethe motors to the shaft 'iS for transiaterymotion of the latter member.The motion imparted to the shaft its is such as to mn output voltage oithe transducer 25 approach the voltage from the converter 2.5.

Still referring to FGURE l, precise, discrete increments of travel ofthe worktahle it? are sensed by a position sensing unit generallyindicated at The unit includes a sensing head Sti, a null tector artnull compensator The sensing head is po immediately below a measuringbar attached to .f slide It is mounted on a slide 3S fitted to a ti onthe carriage l2. The measuring bar "is and are aligned with thedirection ol' motion of i on the carriage l2. Movement of the sensinghead along the track @to is effected by means of a sensing head driveunit 42 acting through a shaft 44.

The measuring bar 3e is provided with lands do periodically disposedalong its length. rthe sensing head is used to detect the passage of theindividual lan s when the lands 45 assume certain positions wi to thesensing head. The false null compensator connectcd to the output of thedetector 32, emits an output signal each time a land da arrives at aunique position over the sensing head 30. More particularly, in theembodiment of our invention described herein, a sig al i emitted bythecompensator 34 each time a land do arrives at the true null position,i.e., centered over the sensing head 30.

rThe lands 46 are centered a convenient distance apart, eg., one inch,which is the range of the tine position he the 6 control. in order tovary the stopping position of the table l@ over this range, the positionot the sensing head is adjusted by the drive unit 42 at the beginning ofeach taule-positioning operation.

More speciiically, as seen in FiGURE l, the power to the drive unit 42is supplied by a control unit i8 which controls the magnitude andrelative polarities of currents in the windinvs of a motor (not shown)contained in the unit d2. The motor is mechanically coupled to amicrometer screw (not shown) which is connected tothe shaft Rotation ofthe micrometer screw by the motor causes the shaft ad and sensing head3G to traverse to the lctt or right, depending on the direction oirotation. The direction of rotation of the motor, in turn, depends onthe polarity of the current supplied from the control unit The inputsignal to the control unit is an error voltage derived in a well-knownmanner from a summing unit Sii which compares the output voltage of aposition transducer 52 with a line position voltage obtained from themaster control unit 22 by way of a digital-to-analog converter 54. Thelatter voltage corresponds to the desired position of the sensing head3'0. The output. voltage of transducer 52 represents the actual positionof the sensing head 3o, and the control unit 4S causes the sensing head3d to `be traversed in the proper direction to reduce the output voltageof the summing unit and thus reduce the diilerence between the actualand desired positions or the sensing head.

Operation of the positioning system of FEGURE 1 is as follows. Assume areference or Zero point on the carriage l2 from which positions of theworlitable it) are measured. The instructions on the tape passingthrough the tape reader 26 are in terms of distance ot' a referencepoint on the table itt from the zero point on the carriage. The mastercontrol unit 22 transmits the desired position to the digital-to-analogconverter 23, and the table control unit 2e then initiates traverse ofthe table lil in response to a start signal fed to an input 2612.

At the same time, the control unit 22 tr smits to the converter 54- thedigits to the right or the decimal place, i.e., fraction of an inch, inthe instruction from the p o reader 2o, In response to a start signalfrom the master control unit, the control unit 48 then commencesadiustment of the sensing head 3o in the above-described manner. Morespecically, the sensing head Sil has a Zero position on the carriave i2.When it is in this position, the respective lands 46 on the measuringbar 36 pass through the true null position at integral inch distances ofthe table 10 from its reference position. In order to position theworktable at other than integral inch tances, the sensing head Sii ismoved distance from its reference point equal to the fractional part ofan inch in the desired table position.

Next, assume leftward motion of the Worktable 10 (FIGURE l), with theland 45a in the true null position over the sensing head 30 when thetable reaches its desired position. A level detector 55, connected tothe output of the summing unit 24, emits a signal when the error voltagecorresponds to a positional error of less than one inch for the table10, i.e., after the land 4Gb has passed the true null position butbefore the land 45a has reached the speed-reducing offset null position.This range is almost a full inch, since the latter null position isimmediately prior to the true null position of the land 46a.

yThe output signal of the detector 55 is used to open a gate 56 betweenoutputs 34a and 34b of the compensator 34 and inputs 26C and 26d,respectively, of the table control unit 26. When the land 46a passesthrough the rst or speed-reducing null position, a signal from theoutput 34a causes a reduction in table speed. At the same time, a signalfrom an output 34C is discontinued and the null detector 32 reverts toits normal state. When the land 46a arrives at the true null position, asignal from the detector 32 is emitted from the compensator output 34baces,

l through the gate 56 to the table control unit input 26d to stop thetable.

Referring now to FIGURE 2, the sensing head 30 has the shape of an Ecore with projections 39a, 3tlb and 39e. A coil S7, having an inductanceLa, is formed on the projection 3G51, and a coil 58 with an inductanceLb is formed on the projection Sab. Assuming that the coils 57 and 58are identical, the inductances La and Lb, which depend on thereluctances of the magnetic paths through the coils, will be equal whena land 46 is centered over the projection 3de, as shown in FIGURE 2. Asnoted above, this is the true null position of a land.

The manner in which the positions of the respective lands 46 over thesensing head 30 are ascertained is illus trated in FIGURE 4. As showntherein, the null detector 32 includes a bridge circuit having as itsarms the inductances L, and Lb and a pair of resistors R1 and R2. Thebridge is powered from an alternating current source indicated at 66.The bridge output voltage is taken from a pair of junctions 62 and 64and amplified by an amplitier 66 before rectication by a rectiier 68. Alevel discriminator 69, such as a Schmitt trigger or the like, emits asignal from an output 69a whenever the output voltage of the rectifier63 is below a predetermined level and a signal from an output 6917whenever the rectifier voltage is above this level. A signal from theoutput 69a indicates balance or null of the bridge circuit, and a signalfrom the output 69b indicates absence of a null condition.

Illustratively, R1 equals R2, and therefore balance of the bridge willtake place whenever La equals Lb. Since the coils 57 and 58 areidentical, the latter condition will occur, for example, in the truenull position of FIG- URE 2.

In order to compensate for departures of the values of the variousbridge parameters from their nominal values, We have included apotentiometer 70 whose tap 70a is connected to the voltage source 60.Adjustment of the tap 7%1 changes the relative resistances on the twosidesl of the bridge and thus changes the effective values of R1 and R2.The potentiometer 70 may be adjusted by tirst setting the sensing head30 in a true null position and then moving the tap 7tlg to provide anull in the output of the rectier 68.

Still referring to FIGURE 4, our invention includes the addition ofcertain elements to the bridge circuit therein to cause a null output ata position other than the true null position of FIGURE 2. Morespecifically, the resistor R2 may be a potentiometer provided with a tapR261, The tap is connected to a resistor R3 in turn connected to agrounded junction 72 at the far end of the coil 5S by a switch Sl. Whenthe switch S1 is open, balance of the bridge circuit will occur at thetrue null position of FIGURE 2, as described above.

When the switch is closed, balance will occur at an oiset true nullposition instead of the true null position. As seen in FIGURE 6, thefirst otset true null condition occurs immediately before the table 10and measuring bar 36 attached thereto arrive at a true null position.Assuming that Rl plus the portion of the potentiometer 70 connectedthereto is substantially equal to R2 plus its associated portion of thelatter potentiometer, the relationships of the position of the tap R2@and the resistance of the resistor R3 to the inductances L,l and Lb ofthe coils 57 and S3 at the iirst oiset null position are given by,

a is the fraction of the resistance R2 between the tap R2a andthe coil58,

Expressions l 2 neglect the er'ect of the mutual inductance M of thecoils 57 and 58. The mutual inductance may be accounted forapproximately by substituting (La-M) and (LV-M) for L, and Lb,respectively.

An important advantage of our oitset null circuit is its simplicity. Itrequires only the addition ot the resistor R3 and switch Sl and the useof a potentiometer as the resistor R7.. Moreover, the values of R2 andR3 may be readily varied to change the positions ot the oiiset nulls.Another advantage is the ease with which a nurnber of offset circuitsmay be switched into and out of operation to provide for differentoffset conditions at different intervals of travel of the lands 46(FIGURE l).

rurning now to FIGURE 5, the switch S1 includes a pair of transistors 76and 77 connected in reverse parallel between the resistor R3 and thejunction 72. Thus, the emitter 76a and collector 77C are connected tothe resistor R3 while the collector 76e and emitter 77a are connected tothe junction 72. The transistors are shown as p-n-p transistors, andtherefore their bases 76b and 77b are positively biased from a biassource (not shown) by way of resistors R4 and R5, respectively. A pairof series resistors R6 and R7 are connected between the transistor basesand the input terminal 78 of the switch S1. A resistor R3 is connectedbetween the terminal 78 and ground.

j The bias voltage and the resistances of the resistors RdeR are chosenso that in the absence of an input signal at the terminal 78, thepotentials of the bases 76b and 77b will always be greater than theinstantaneous voltage at the tap Rza. Thus, both transistors 76 and 77will be cut ott and the switch S1 will be open.

When an input signal in the form of a negative voltage is applied to theinput terminal 78, base-emitter conduction takes place in both thetransistors 76 and 77, thereby saturating the transistors and providinglow impedance paths through them from the resistor R3 to ground. Theinput signal at the terminal 78 thus effectively closes the switch Sl toplace lthe null detector 32 in the lirst oiset condition.

A second switch S2, similar to the switch S1, is connected betweenground and a resistor R9 tied to the tap REQ. Thus, when an input signalis applied to the input terminal Sti of the switch S2, the detector 32is placed in the second offset condition. The posi-tions of the rst andsecond oi'set nulls corresponding to the offset conditions aredetermined by the setting of the tap R2a and the resistances of theresistors R3 and R9, as described above.

Still referring to FlGURE 5, the false null compensator 34 includes aseries of nor elements 84-101 connected in a logical circuit whichoperates in the manner described below. The voltages `at the outputs 69aand 6% of lthe level discriminator 69 altern-ate between two levels,ydepending on the existence or `absence of a null.

Riese levels correspond to the binary digits 0 and l.

l and the output 69h a G; when there is no null, the Voltaees at theoutputs 69a and 6911 will correspond to 0 and l, respectively.

Each of the nor elements d-I is an inverter which provides an outputsignal, i.e., a l, when each of its input signals is 0. Thus, the logicequation for each of the units is,

W is the output level, and X, Y and Z iare the input levels.

T he circuit of FIGURE 7 is `a convenient embodiment of annees@ thistype of element. As shown therein, a transistor 192 has a collectorZitZc connected to a source of negative bias (not shown) through aresistor R10. The b-ase ltlZb is connected to the positive bias sourcethrough a resistor Ril. The X, Y and Z input signals are applied inseries with resistors R12, R13 and R14, across the base tZb and thegrounded emitter 10201. The output W is taken from the collector M920. Al is represented by a negative voltage level and a by O Volts. lt willbe apparent that, in the absence of input signals X, Y and Z, i.e.,X=Y=Z=0, the transistor to2 will be biased to cutoff by the positivebias applied to the base. The output voltage will then be the potentialof the negative bias, i.e., a. l. If any of the inputs is a 1, anegative voltage will be applied to the base 162]? and the transistorjitlZ will conduct, thereby grounding the collector ltZc and reducing W,the voltage at the output, to 0.

Referring once again to FlGURE 5, the output voltages of the elementsSei-lili are designa-ted by the letters A-M and P-T, respectively.Voltages at the outputs e 69a and 6% of the discriminator 69 aredesignated herein as N and N, respectively. Following this convention,an input of the nor element de is A, and an input of the element Se isB. The cro-ss coupling of these two elcmcnts makes a memory 84--85 outof them, since, if the "f' output of one of them is a l land the other a0*, this condition will rem-ain as long as no co-unteracting inputsignal is applied. In particular, in order to reverse the state of thememory, an input signal, i.e., a l, is applied to the element whoseoutput is 1, and the output is forced to 0. The output of the otherelement, which was a 0, will then go to l. This 1 will maintain theoutput oi the rst element at 0, after the counteracting input signal isremoved. The compensator 34- also includes memories S-S, tl-Q, @I-9d,'9o-97 and 99-100, similar to the memory Sti-S5. The states of thememories m-ay be considered on when the outputs of the upper norelements (FIGURE 5) are ls, and off when they are Os.

An additional input to the elements S and S7 is the output of theelement @it (G). The element 3S has as one of its inputs the output ofthe element 101 (T), and another is a switch N3 connected to a source ofl voltage, illustratively indicated as a battery 194. The switch 193 isalso connected to inputs of elements S7, @il and 93. The inputs of thenor element Se are B and and the output C of -this element is an inputof the element 3S. N and E are inputs for the elements S9 whose output Eis an input for the element 9i. Similarly, I- and are the inputs `forthe element 92, and P and for the element 98, While the outputs I and Qof these elements are inputs for the elements 9d and ltli, respectively.En like manner, K and N are the inputs for the element 9S, and theelement lol. has S and N as its inputs, the output L of the element 9Sbeing one of the inputs of the element 97.

Still referring to FlGURE 5, it is seen that M is an additional inputfor both the nor elements 9d and 93, and A is an additional input forthe elements and 99. A is also the voltage at the output Stia' of thecompensator 34- connected to the switch terminal of the null detectorS2.. ri`he voltage G appears at .the output Sec connected to the switchterminal 78. The outputs Sli-a and 341'), connected to the ltablecontrol unit 2o (FlGURE l) by Way of the gate So, are lvl and Q,respectively.

rl`he `compensator Se is initially set by momentarily closing the switchw3. This places the memory 34--55 in the on state and the memories397-33, Ml- 9i and Xi-9d in the oif state. In the on state of the memoryA is l, thereby imposing the off condition on the memories 9i97 and-ltlt Also, as pointed out above, a l is denoted by a negative voltage,and therefore the switch S2 connected to the output Sed is closed,thereby placing the null detector 32 in the sec- 10 ond offsetcondition. The conditions at the outputs of the various nor elements aregiven on line 1 of FIG- Assume that, with leftward motion of theworktable l@ and measuring bar 36, motion begins with the land to theleft of the true null position, as seen in FIG- URE 6, and before thenext false null position of the measuring bar 3d is reached. With thenull detector 32 in the second oifset condition, the bar 36 will passthrough the false null `and first oifset false null positions withoutemitting an output signal. Ho-wever, a null will be indicated at thesecond offset false null position, or if that null is not detected,there will be an indication when the second offset true null is reached.

The null indication Will be in the form of a change of N from to l andfrom l to 0. As seen from FIG- URE 5, both inputs of the element 86 willthen be 0, and C will therefore go from 0 to l, thereby shifting thememory 87-88 to the on condition. The circuit conditions will then be asindicated on line 2 of FIGURE 8.

As the measuring bar 36 (FIGURE 6) leaves the above null position, Nrever/ts to O and to 1. With both inputs of the nor element S9 at 0, Fgoes to 1, thereby imposing the on condition on the memory gti-9i. G istherefore l, and thus the memories 84- and 37--83 are switched to theolf condition, as indicated on line 3 of FlGURE 8. Also, it is seen thatthe changes at this point cause opening of the switch S2 and closure ofthe switch S1. Consequently, the null detector 32 is in the lirst orspeed-reducing offset condition.

The measuring bar 36 continues its movement until the rst offset true.nu-il position is reached, as indicated in FiGURE 6. As seen in FIGURE5, N once again changes lto l and to 0. Both inputs of the nor element9E are therefore (l, and I changes to 1. This causes a shift of thememory 93-94 to the on condition. The conditions of the various norelements at this position are `shown on line i of FIGURE 8.

When the measuring bar 36 leaves the first offset true null position, Ngoes to 0 and to l. Both inputs of the element 9S are now '0, and Ltherefore shifts to l, thereby changing the memory 9d-97 fro-m the ofito the on state. M is therefore 1, and, if the gate Se is open, aspeed-reducing signal will be transmitted from the output 34a to thetable control unit 26 (FIGURE l). At the same time, the memories iii-31and 935-94 are returned to the off state, as indicated on line 5 ofFIGURE 8. The detector 32 is now in its normal condition, withoutoffset, and thus ready to detect the forthcoming true null.

Assuming that the speed-reducing signal has been transmitted to thetable control unit 26, the measuring bar 34 continues at greatly reducedspeed toward the true null position of the land ida (FIGURES 1 and 6).At the true null position, N goes to l and to C, the conditions reachedby the various nor element outputs are shown on line 6 of FiGURE 8.Thus, with both inputs of the element 9S at i), Q goes to 1 and a stopsignal is transmitted from the output 34h to the table control unit 26.The worlctable 1G (FIGURE l) then coasts to a stop, as described above.

It will be noted that the above sequence of changes in the conditions ofthe nor element outputs in FIGURE 5 will be the same whether or not thesecond offset false null position is detected. If it is detected, thedetector 32 will be in the rst offset condition as the measuring bar 3dpasses through the second oset true null position, and therefore thelatter position will not be detected. If the second offset false nullposition is not detected, the detector 32 will remain in the secondodset condition until it reaches the second offset true null positionwhich will then be detected. Thus, for each unit of travel of themeasuring bar 36, either the second offset false null position or thesecond offset true null position will be detected. The offset true nulland and first offset false null positions are never detected, becausethe detector 32 is in the second oiset condition when it passes throughthese points. The iirst oitiset true null position and the true nullpositi are always detected. Thus, by switching the detector into itsoffset and normal conditions, the compensator 3d converts the detectionot true nulls to a simple counting operation whereby every third nullindicated at the output of the null detector 32 is a true null. indeed,as will be seen, the compensator 34 operates as a ring counter when usedto count a succession ot true nulls.

More specifically, assuming that the gate 56 was closed, and thereforespeed-reducing and stop signals were not transmitted to the tablecontrol unit 26, arrival of the land 56a (FIGURE 6) at the true nuilposition shifts Q to l, as stated above, and the memory g-idtt istheretore switched to the on state. The compensator conditions at thispoint are reflected on line 6 of FGURE 8. 'When the land 46o leaves thetrue null position, accompanied by a change of N to O and to i, bothinputs of the nor element itii will be 0. T will thus be i, therebyshitting the memory tid- 85 to the on condition and placing tie detector32 in the second oset condition. With A at l, the memories @t3- 57 and99--ititi are turned ofi. 'ihe Voltage conditions in the compensator 34will thus be as indicated on line 7 of EGURE 8. Line 7 is identical toFIGURE 1, and therefore the above cycle ot changes in the comparator 34will take place as the measuring bar 36 goes through its next unit oftravel over the sensing head 30.

Proper operation of the circuit ot FIGURE 5 requires that the initialconditions be imposed at or prior to the last second offset true nullposition preceding the nal true null or stopping position of themeasuring bar 36 (FIGURE 6). This is assured by the fact that, in orderto maintain uniform stopping conditions, the table control unit 26causes the table to back off (to the right in FEGURES 1 and 6) toprovide a minimum travel of onehalf inch whenever the requireddisplacement of the worktable iti is less than that ligure.

it will be apparent that other circuits than the ones specificallydescribed in FIGURE 5 may be made to operate in the above manner. Forexample, the outputs or" the discriminator 69 may be passed throughdilierentiators which provide pulses when the discriminator shifts fromone state to the other. The pulses may then be counted in a pulsecounter. Also, if the counting ot successive true nulls is not required,the nor circuits 99, iti and iti may be eliminated. In such case, theswitch i313, which may take any suitable form, may be momentarily closedby the signal from the level detector 55 which opens the gate 56.

Another modiiication of the circuit of FiGURE 5 may be made when thereis a requirement that the successive lands of the measuring bar 36(EEGURE l) be counted and no requirement that speed-reducing and stopsignals be emitted by the compensator 3d. This permits elimination ofthe offset circuit associated with the first oitset condition of thedetector 32. The compensator 3d may then be simpliiied by theelimination of six of the nor elements therein, since there will be onlytwo instead of three nulls to be counted for each unit of travel of themeasuring bar. The principle of operation, however, will still be thesame. Each true null will be detected by the detector 32 as well as oneor the other of an oiset false null and an offset true null.

With the circuit of FIGURE 5, our positioning system is adapted tooperate with the table iti (FGURE l) approaching from the right. Thesystem may be adapted for approach from the left as well by adding aSecond set of oset null circuits comprising resistors connected betweena tap on the resistor Ri and ground, with switches similar to theswitches S1 and S2 connected to switch the resistors into and out of thecircuit. Then, for left- Ward movement of the table i0, the oifsetcircuitry shown in FGURE 5 will be connected to the alse nullcompensator 34, and tor rightward movement, the otiset circuit on theother side bridge circuit will be connected to the compensator. will beapparent that our invention is not limited to straight line motion butmay also be used to control the angular positions ci rotat'ng members.in the latter case, the measuring bar .ad {EiGURE l) wiil have anarcuate or circular configuration.

FUURE 9 illustrates another arrangement which may be used to shift anoffset null from one side of a true null position to the other. Thebridge circuit has been modilied to a potentiometer as the resistorPotentiometer taps Ria and R251 are connected to the end terminals of apotentiometer Riti. rthe tap Ritiri of the potentiometer is connected toground by way of the switch Si.

`Assuming that Ri equals R2 and that the taps iia and Rim are set at thesame points along the potentiometers 1 and R2, respectively, there is nooffset in the circuit when the switch Si is open. When the switch isclosed, the null position of the bridge will be offset to the right orleft ot the normal position, depending on which side ot the electricalmidpoint of the potentiometer Riti the tap Rita is on. in other words,the otliset position can be shifted from one side of the normal positionto the other by moving the tap itin from one side ot the electricalmidpoint of the potentiometer to the other. The distance of the offsetposition from the normal position depends on the electrical distance ofthe tap 1Go from the midpoint of the potentiometer. The tap ida may bemoved automatically to shift the offset position as required in thepositioning system described above or in other systems in which thebridge is used.

it will thus be seer that the objects set forth. above, among those madeai., .rent from the preceding description, are efficiently attained and,since certain changes may be made in the above construction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdr'wings shall be interpreted as illustrative and not in a limitingsense.

is also to be understood that the following claims arc intended to coverall of the generic and specific features of t scope of the inventionwhich, as a matter ot language, might be said to fall therebetween.

We claim:

l. in a positioning system adapted to position a rst elet 1ent withrespect to a second element, said system having a position controlincluding a serrated bar mounted on one of said elements and a magneticsensing head mounted on the other of said elements, said bar and sensinghead being so disposed that the serratio of said bar successively passover said sensing head du g movement of said lirst element, and a nulldetector adapted to emit an output signal when a serration arrives tsecond positions with respect to said sensing sai system also includinga coarse positioning controt adapted to bring a selected serration intothe field of sensi' g head, the improvement characterized by a false nsupp ssor adapted to eliminate random detection aid second positions,said suppressor including detector to emit signals at third and fourthpositie s oA id serration instead of said iirst and second positions, sathird and fourth positions being reached by said serrations after saidsecond position and before said first position, means adapted to connectsaid lirst otiset means to said detector before a serration reaches saidthird positi n and disconnect said tirst offset means when the nextsignal is emitted by said detector, whereby said detector emits onesignal when said serration passes through said third and fourthpositions and another signa when said serration passes through said irstposition.

2. 'the combination defined in claim l in which said suppressor has afirst output terminal, means for indicating sa id i3 signals from saiddetector at said first terminal, and means for suppressing at said firstterminal signals other than those occurring when serrations pass throughsaid first position.

3. The combination defined in claim 1 in which said sensing `headcomprises a pair of inductors whose inductances arefunctions of thepositions of said serrations relative thereto, saiddetector comprising abridge circuit including said inductors, the parameters of said bridgecircuit being such that a null is obtained whenever one of saidserrations is in said first position, and including means lfor emittinga signal whenever the unbalance voltage of ysaid bridge circuit is belowapredetermined level.

4. The combinationdefined in claim 3 in which said offset meanscomprises a circuit element which when connected to said bridge circuitalters the balance conditions thereof s as to provide 4nulls at saidthird and fourth positions.

5. The combination defined in claim l including second offset meansadaptedwhen connected to said detector to cause said detector to emi-tsignals at a fifth position intermediatesaid second and third positionsand a sixth position intermediate said ,fourth and first positions,means for connecting said second Offset means to said detector whenVsaid first offset means is disconnected and disconnecting saidsecondoffset means when the next signal following disconnection of said secondoffset means is emitted bysaid detector, andmeans adapted to reduce therelative speed of said elements when said second offset means isdisconnected.

6. A position sensing system adapted to sense the position of la hrstelement with respect to a second element, said system comprising, incombination, a serrated bar of .magnetic material mounted on said firstelement, a sensing `headrnounted on` said second element, said bar andsensing head ,being sordisposed with respect to each other thatsuccessive serrations on said bar move past said sensing head duringrelative movement of said elements, said sensingghead comprising a pairof inductors whose inductances are functions of the distances of saidserrations therefrom, a detectorincluding said inductors and adapted toprovide an output signal when a serration arrives at a first positionandra second position with respect to said sensing head, said detectorincluding first offset means adapted when connected to said detector tocause said detector to emit signals when said serration is at third andfourth positions instead of said first and second posi'ions, said thirdand fourth positions being reached in order by said serrations aftersaid second position and before said first position, and a false nullsuppressor adapted to compensate for random detection of arrivals ofsaid sei-rations at said second position, said suppressor comprisingmeans adapted to connect said offset means to said detector after aserration has reached said first position and means adapted todisconnect said offset means when the next signal is emitted by saiddetector.

7. The combination defined in claim 6 including second offset meansadapted when connected to said detector to cause said detector to emitsignals when said serration is at a fth position intermediate saidsecond and third positions and a sixth position intermediate said fourthand first positions instead of at said first, second, third and fourthpositions, means adapted to connect said second offset means to saiddetector when said first offset means is disconnected therefrom and todisconnect said second offset means when the next signal following saiddisconnection is emitted by said detector and means for emitting anoutput signal when said second offset means is disconnected.

8. The combination defined in claim 6 in which said detector includes abridge circuit comprising a source of electrical potential having firstand second terminals, a rst resistor connected to rst terminal andconnected in series with one of said inductors between said terminals,

id a second resistor connected to said first terminal and connected inseries with the other of said inductors between said first and secondterminals, said offset means comprising a third resistor, a tap on saidfirst resistor and a switch adapted to connect said third resistorbetween said tap and said second terminal.

9. The combination dened in claim 8 in which said suppressor is adaptedto close said `switch when a serration reaches said first position andopen said switch when the next signal is emitted by said detector.

l0. The combination defined in claim 7 including a first switch adaptedto connect said first offset means to said detector, means for closingsaid first switch after a first serration has passed said rst positionand before the next serration has reached said fourth position, acounter adapted to count said signals from said deteclor, said counterhaving a first output adapted to emit a signal in response to the nextsignal from said detector after said ciosing of said first switch, asecond switch adapted to connect said second offset means to saiddetector in response to said signai from said first output, said counterhaving a second output adapted to emit a signal when said next serrationpasses through the next position at which said detector emits a signal,said second switch being adapted to disconnect said second offset meansin response to said signal from said second output, whereby both saidoffset means are disconnected from said detector, said counter having athird output adapted to emit a signal when said serration arrives at thenext position at which said detector emits a signal following saidsignal from said second output.

ll. The combination defined in claim lO in which said counter is a ringcounter and includes a fourth output adapted to emit a signal when saidserration leaves said last-mentioned position, said first switch beingadapted to connect said first oliset means to said detector in responseto said signal from said fourth output.

l2. A detector adapted to emit signals corresponding to the passage ofscrrations on a magnetic bar through a first position with respectthereto, said detector comprising first and second inductors whoseinductances are functions of the positions of said serrations withrespect thereto, a source of alternating potential having first andsecond terminals, a first resistor connected to said first terminal andconnected in series with said first inductor between said first andsecond terminals, a second resistor connected to said first terminal andconnected in series with said second inductor between said first andsecond terminals, said first resistor having a tap thereon, a thirdresistor and a switch connected in series with said third resistorbetween said tap and said second terminal.

13. The combination defined in claim 12 in which, when said switch isopen, said detector emits signals when said serrations pass through asecond position with respect thereto and when said switch is closed saiddetector emits signals at said first position and when said serrationspass through third and fourth positions after said second position andbefore said first position instead of at said first and secondpositions.

i4. The combination defined in claim l2 including an E core of magneticmaterial, said E core having first and second outer projections and aninner projection, said inductors being coils wound around said first andsecond projections, said first position of a serration occurring when aserration on said bar is positioned substantially directly opposite saidinner projection and equidistantly from said first and secondprojections.

l5. A detector adapted to emit signals corresponding to the passage ofserrations on a magnetic bar through a first position with respectthereto, said detector comprising an E core of magnetic material, saidcore having first and second outer projections and an inner projection,a first coil wound on said first projection, a second coil wound on saidsecond projection, a source of alternating current potential havingfirst and second terminals, a first resistor connected to said firstterminal and connected in series with said first coil between said firstand second terminals, a second resistor connected to said first terminaland connected in series with said second coil between said first andsecond terminals, a tap on said irst resistor, a third resistor and aswitch connected in series between said tap and said second terminal,the resistances of said resistors and the inductances of said coilsbeing such that when one of said serrations is at said position and saidswitch is open there is a minimum in the voltage between the junctionsof said rst resistor and said first coil and the junction between saidfirst resistor and said second coil.

16. Fthe combination defined in claim 15 including means adapted to emitan output signai whenever the voltage etween said junctions is beiow apredetermined level.

17. in apparatus adapted to emit output signals corresoon ing to thepassage of serrations on a bar of inagnetic material into a irstposition with respect to sensing signers when said serrations pass intosaid rst position, said 'etector also being adapted to emit signals inrandom fashion when said serrations pass into a second position withrespect to said sensing head, the combination of odset means adapted tocause said detector to emit signals when said serrations pass throughthird and fourth positions with respect to said sensing head instead ofsaid rst and second positions, each of said serrations passing throughsaid third and fourth positions after passage through said secondposition and prior to passage through said first position, a compensatoradapted to activate said offset means after a serration reaches saidfirst position and inactivate said offset means after the next signalfrom said detector, said compensator being adapted to ernit one outputsignal from each two signals emitted by said detector.

18. 'he combination defined in claim 17 in which said compensator is acounter adapted to count said signals from said detector, said counterhaving a first output adapted to emit a signal in response to the signalemitted by said detector when a serration passes through said third andfourth positions, said apparatus including means lf3 adapted toinactivate said offset means in response to said signal from said firstoutput of said counter, said counter also being adapted to emit a signalin response to the next signal emitted by said detector afterinactivation of said ofiset means.

19. The combination defined in claim 18 in which said counter is a ringcounter adapted to emit an activation signal when a serration passesthrough said first position, said apparatus including means foractivating said offset means in response to said activation signal.

20. A detector adapted to emit signals through the passage of serrationson a magnetic bar through a first position with respect thereto, saiddetector comprising first and second inductors whose inductances arefunctions of the positions of said serrations with respect thereto, asource of alternating potential having first and second terminals, afirst resistor connected to said first terminal and connected in serieswith said first inductor between said first and second terminals, asecond resistor connected to said first terminal and connected in serieswith said second inductor between said first and second terminals, saidfirst and second resistors having taps thereon, a po tentiorneter whoseend terminais are connected to said taps on said first and secondresistors, means connecting the tap on said potentiometer to said secondterminal, and sensing means sensitive to the potential between thejunction of said first resistor and said first inductor and the junctionbetween said second resistor and said second inductor.

References Cited in the le of this patent UNITED STATES PATENTS

